1. Field of the Invention
The present invention relates to a manufacturing technique for three-dimensional parts based on computer data representing their shape, and a device embodying this technique. Creating parts by the transformation of successive volumes (generally thin layers) of a raw material in an initial and secondary state by means of a device inducing said transformation, through the repetition of a cycle specifically involving a transformation phase of the material in at least one part of the working field performed by the device inducing the transformation, and a coating phase of the transformed material by the material which is not yet transformed, is known art.
2. Description of the Prior Art
Most of the so-called xe2x80x9cRapid Prototypingxe2x80x9d machines employ this type of technique, in particular stereolithography machines, which use a photosensitive liquid raw material (which may be polymerized or reticulated) combined with a lighting device (using ultraviolet laser scanning, for example), and the so-called powder sintering machines, using a raw material in the form of a powder, which may be locally sintered by thermal means (infrared laser scanning, for example).
Numerous patent applications have been filed in relation to this type of machine, in particular in the field of stereolithography. A complete description of the technique is provided in document EP 0361847, and similar techniques are also described in documents EP 0450762 and EP 0484182. In document EP 0287657 a detailed description is provided of a powder sintering technique, as well as a description of the device for the implementation of the technique.
One feature common to the stereolithography and powder sintering techniques is that they require the implementation of means to spread the raw material in the form of thin layers (to perform the coating phase), automatically, rapidly and homogeneously. In general, the coating phase consists in sweeping the surface of the working field with the aid of at least one elongated device called a recoater blade, which is placed in movement over the working field. Depending on the case, the recoater blade transports the material used for the coating, or simply levels off a previous deposit of material.
Numerous devices have been designed to embody the recoater blade. For example, in the field of powders, in document EP 0287657, there is a description of a recoater blade composed of a dynamic roller which rotates around it axis, and is placed in parallel to the working field; the lower part of the roller has the same elevation as the working field. This roller also travels carrying a volume of material located upstream and gradually spreads a new layer of material downstream, thus coating the previously transformed material.
With respect to liquid raw materials, two types of techniques can be distinguished: the specific coating process is ensured in advance on the working field by coating media (chute, spray, immersion, etc.), and the recoater blade simply levels the free surface of the liquid by means of a horizontal sweeping movement, or the recoater blade performs the coating and leveling functions simultaneously. The second mode of operation involved the transport by the recoater blade (or by associated attached media) of a volume of material sufficient to coat the zones, which have just been solidified.
Various devices have been designed for the creation of the recoater blade, for example, a simple rigid blade (cf. Document EP 0361847), whose cross-section may have a specific shape, or the association of two rigid elements (document EP 0484182), which are attached to brush elements laid out in staggered rows. These initial devices have not been completely satisfying.
In fact, when using a so-called xe2x80x9csimple recoater blade,xe2x80x9d such as, for example, a blade (or any other more or less flexible equivalent), whose lower part comes in contact with the free surface of a liquid, in parallel to said free surface, stress is generated in the area of contact between the blade and the liquid. This local shear stress is transmitted to the liquid located below the free surface, at a depth much greater than the thickness of the placed layer.
In the case of the resins traditionally used in stereolithography, whose behavior is similar to that of a Newtonian liquid, with viscosities of several thousand centipoises, the recoater blade may generate movement in the liquid at a depth which may range from several millimeters to several centimeters.
The portions of solid or semi-solid material transformed during the preceding cycles represent obstacles to the liquid in movement, which translate into strong variations of the liquid""s flow condition. Therefore, near the edges of the section being produced, flatness defects appear in the layer that was just spread by the recoater blade. A schematic illustration of these faults is provided in document WO 95/15842, where meniscuses (concave or convex) are depicted near the liquid-solid transition areas.
These meniscuses cause geometric faults on the parts created, and, once solidified, represent an additional impediment for the placement of the following layer, which finally, translates into an amplification of defects, incompatible with the coating device (risk of pulling up the layers, material accidents, etc.).
In order to limit these problems, it is sometimes possible to slow the speed of movement of the recoater blade, or impose a setting period prior to proceeding with the following transformation phase, but these solutions result in a substantial increase in part manufacturing time, leading to increased production costs, which may be prohibitive.
The solution proposed in document WO 95/15842, does not call into question the use of so-called xe2x80x9csimple recoater blade,xe2x80x9d but rather consists in creating a covering structure which closely follows the shape of the parts and has the effect of distancing the meniscus problems from the areas of said parts. This clever technique allows for the use of a xe2x80x9csimple recoater bladexe2x80x9d composed of a thin, elongated, flexible part, and allows for very short time frames for the coating phases. The provision of the material necessary for the coating phase takes place by the transport of a type of wave of material, formed at the start of the recoater blade""s movement. The volume of material included in said wave is progressively consumed by the effect of the combination of gravity and movement.
Another solution, described in document WO 96/23647 consists in using a so-called xe2x80x9cactive recoater blade,xe2x80x9d in this case, a dynamic roller which moves in a xe2x80x9ccounter-rotationxe2x80x9d direction, associated with a type of barrier, composed of a rectilinear blade, whose lower side is located a short distance from the upper part of the roller. A roller turning in a xe2x80x9ccounter-rotationxe2x80x9d direction is such that any point located on its periphery when it passes the point of its trajectory closest to the working field, has a relative tangential speed with respect to the axis of rotation moving in the same direction as the speed of movement of the rotational axis of the roller with respect to the working field.
The operation of this device is illustrated in the figures attached to this document, for the case in which a prior deposit of material has take place downstream. The barrier forms an obstacle to the free circulation of the liquid located downstream of the roller, allowing for the regulation of the thickness of the film formed on the upstream portion of the roller. The film""s free surface meets that of the layer formed upstream, thus forming a sharp lap-back point in proximity to the lower portion of the roller. This lap-back point defines the height of the free surface of the material placed upstream, and since it is constantly fixed with respect to the roller axis, the layer may be leveled.
A detailed analysis of the physical phenomena involved is proposed in the document in order to explain the advantages of such a device with respect to a single blade (simple recoater blade), in particular with respect to the interactions with the liquid located near the recoater blade. In particular, it appears that the xe2x80x9ccounter-rotationxe2x80x9d direction of the roller""s rotation strongly attenuates such interaction, while on the other hand, a movement in the non xe2x80x9ccounter-rotationxe2x80x9d direction, which will be called the xe2x80x9crolling directionxe2x80x9d hereinafter, would induce effects of pressure resulting in the placement of a non-homogenous layer.
Different variations are proposed, in particular concerning the cross-section of said barrier and its orientation. Other variations aiming at remedying the undesirable effects of the potential accumulation of material near the blade are addressed: using an endless screw removal device, or a blade with a channeling system. Finally, two feed methods are proposed for the device: either the layer of material is deposited downstream in advance, and the recoater blade simply corrects the residual flatness defects, or the recoater blade transports a volume of liquid sufficient to provide the quantity of material necessary for the coating (as in the solution of document WO 95/15842).
These exists a range of materials which are of particular interest for Rapid Prototyping, in particular those allowing a way around the primary defect of powders (the creation of porous parts) and of acrylic or epoxy liquid resins (low mechanical strength, fragility, etc.): highly viscous, even pasty materials. These materials may be obtained, for example, by the addition of a high volumetric rate of sediment load (powder), to a bonding agent composed of photosensitive or thermosetting liquid resin. Hereinafter, pastes will be qualified as the category of materials including materials of a very high viscosity (greater than 10,000 centipoises), or xe2x80x9cmarked thresholdxe2x80x9d materials. A xe2x80x9cthresholdxe2x80x9d material is one that does not flow (no gradient) such that the shear stress applied to it does not go beyond a minimum value. A material will be considered as having a xe2x80x9cmarked thresholdxe2x80x9d when the value of this shear stress is greater than 20 Newtons per square meter.
The known solutions for performing the coating phase are not appropriate for pastes, primarily due to their relative xe2x80x9cinsensitivityxe2x80x9d to the effect of gravity.
In fact, first, the materials just transformed by a layer of paste must be coated, which is practically impossible by means of a simple immersion technique. The solution traditionally adopted, consisting of transporting a volume of material with a scraper, placed in movement in parallel to the working field, and progressively depositing this volume of material, located downstream of the scraper, over the lower layers, is relatively difficult to implement with pastes. In order for the spreading to take place, it is indispensable for the volume of material located downstaeam of the scraper, near the lower edge thereof, to come in contact with the free surface of the lower layers. In fact, the volume of material is thus subjected to a strong speed gradient (speed of movement of the recoater blade for the part in contact with the scraper, and no speed whatsoever for the part in contact with the lower layers), generating the shear stress required to obtain the indispensable flow of material upstream. As long as gravity ensures a descending flow of the volume of material located downstream, with sufficient speed to offset the flow of material consumed by the spreading, this indispensable contact may be maintained. However, if the contact is broken, the volume of material upstream is simply transported, without being spread. Now, with pastes, the flow induced by the effect of gravity on the volume downstream of the scraper is very weak (given the high viscosity), and may even by nonexistent if its flow threshold is sufficiently high. Consequently, even if a volume of material is provided downstream of the scraper from the start, which is theoretically sufficient to ensure the desired coating, there is still the risk of xe2x80x9cdetachment,xe2x80x9d or coating defects (formation of xe2x80x9cholesxe2x80x9d in the deposited layer), which are incompatible with the manufacturing technique.
Of course, in order to skirt the problem one could consider performing a prior deposit of material, which the scraper would simply play a leveling role. However, it is not easy to perform such a prior deposit. In fact, transfer means adapted to pasty materials would have to be used (special pumps), in order to ensure their placement over the working field, thus leading to additional costs and complexity, especially if we wish to maintain strict control of the delivery of the material. In addition, so as not to risk xe2x80x9cdetachmentxe2x80x9d a continuous excess of material would have to be supplied downstream. Now, this would imply a progressive accumulation of material on the scraper during its path over the working field, and thus the need to implement means to eliminate the volume thus accumulated.
The technique embodied by the invention allows for the resolution of these problems.
The present invention provides its benefits across a broad spectrum. While the description which follows hereinafter is meant to be representative of a number of such application, it is not exhaustive. As will be understood, the basic methods and apparatus taught herein can be readily adapter to many uses. It is intended that this specification and the claims appended hereto be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.
In order to simplify the following description, we have chosen to implicitly address the specific case in which the working field is horizontal (allowing for the use of expressions such as xe2x80x9cover,xe2x80x9d xe2x80x9cthe lower edge,xe2x80x9d etc.). This rhetorical choice should not be interpreted as limiting the scope of the invention, to the extent that with pastes, the technique embodied by the invention may operate with a non-horizontal working field, and even with a working field presenting a curved surface.
In accordance with one embodiment of the invention, during at least one of the coating phases, said coating phase employs at least one recoater blade placed in movement to perform the various coating phases. Said recoater blade is composed of at least one elongated element called the pusher, whose lower edge covers a surface coinciding with the surface of the working field during the trajectory of the recoater blade over the working field. A type of extruded log of raw material, that is an elongated volume of raw material is formed and placed against the lower edge of the pusher, downstream, with the lower portion of the log being located in proximity to the surface of the working field, and the extruded log of raw material is placed in rotation around its own axis, by means of at least one device called a roller, located downstream of the pusher, with said roller being placed in rotation around an axis in parallel to the recoater blade.
Thus, a rapid recycling of the raw material near the lower edge and downstream of the pusher is ensured, allowing for the resolution of the problem of xe2x80x9cdetachmentxe2x80x9d referenced above, and the extruded log is transported by the pusher over the surface of the working field.
As an advantage, in accordance with the invention, said extruded log of raw material is placed in rotation in the xe2x80x9crolling direction.xe2x80x9d In fact, this direction of rotation allows the material composing the lower portion of the extruded log to move with a speed component opposite that imposed by the movement of the pusher, which contributes to limiting the interaction with the lower layers and thus limiting the scope of the resulting defects (meniscuses).
In accordance with the invention, the extruded log of raw material formed downstream of the pusher may be transported by a non-sliding rolling movement over the working field, by adapting the speed of rotation specific to the raw material to the speed of movement of the pusher. This specific method of transporting the raw materials ensures a relative speed of virtually zero for the material located in the lower portion of the extruded log, with respect to that of the lower layers, which it comes in contact with said lower layers, which contributes strongly to a xe2x80x9cgentlexe2x80x9d placement, as if a film of paste were rolled onto the lower layers. It is sufficient for the portion of material located near the exterior surface of said extruded log to maintain this non-sliding rolling movement in order to obtain the desired effect (potential internal movements of the material within the log, which do not conform to the non-sliding rolling conditions, are acceptable). Pastes obtained by the addition of a sediment load into a liquid bonding agent are generally opaque, and only the exterior surface of the material log is visible. For this reason, in order to obtain the desired beneficial effect, it is considered that the material log should be transported with an xe2x80x9capparentxe2x80x9d non-sliding rolling movement.
In accordance with the invention, in order to supply the recoater blade with the raw material necessary for the coating, we proceed as follows: along the recoater blade""s trajectory, through a feed mechanism, at least one volume of raw material is formed whose upper portion extends above the surface defined by the lower edge of the pusher when the recoater blade is in motion, in such a way that, during its trajectory, the recoater blade levels off said extended portion of raw material and transports it in the direction of the working field to coat the previously transformed material. This feed method allows, in particular, for the movement of the feed zone outside of the working field, and avoids the implementation of mobile feed methods. As a variation, a continuous feed of non-transformed, raw material can also be envisioned.
It is advantageous to use raw materials which, in their initial state, are pasty materials presenting a marked flow threshold. In fact, with such materials, the depth at which movements are caused in the lower layers is limited, since these movements can only take place at a level of minimal stress (the flow threshold). Now, the lower said depth is, the less the risk of the appearance of meniscuses.
In a variation of the invention, the reversal of the direction of movement of the recoater blade takes place at least once to perform two successive coating phases (whether separated or not by a transformation phase). This method allows us to avoid the recoater blade traveling over a recycling path, which would be necessary if the coating phase were always performed in the same direction. Within the framework of this variation of the invention, its is advantageous to form the log of raw material between two pushers laid out in parallel (at least one roller being placed in the space between the two pushers), in such a way that, at the time the movement of the recoater blade is reversed, there is always an extruded log downstream of a pusher. In fact, as described in the commentary on FIG. 3, the log may be detached from the pusher with which it was in contact, and be recovered by the second pusher after the movement is reversed. Thus, it is possible to resolve the need to provide material prior to each coating phase.
In accordance with the invention, the two pushers may be independent or linked by a single piece structure with an inverted U or other shape.
It is not necessary to have more than one roller, since given that it is placed between the two pushers, it may be used in alternation with one or the other, potentially by adapting its direction of rotation to the direction of movement. As illustrated in FIG. 3, it may be advantageous for the implementation of this variation of the technique in accordance with the invention, to create a recoater blade comprised of at least two pushers connected to a common chassis, with said chassis being linked to the recoater blade""s control and direct mechanism by means of a pivot hinge.
As an advantage, an appreciable elevation of the pusher, located downstream (in the direction of movement of the recoater blade) in ensured, with respect to the surface of the working field, so that it is not loaded with raw material, during its passage over the working field, which it levels off the potential meniscuses created during the preceding coating phase. After the reversal of the recoater blade""s direction of movement, the raw material thus accumulated on the pusher runs the risk of being placed haphazardly on the working field, which would lead to the placement of an irregular layer.
The device embodied by the invention may be comprised as follows:
of means to induce the transformation of the raw material on a working field.
of means to perform the coating phases using at least one recoater blade comprised of at least one pusher, said pusher being comprised of a mechanical element, elongated in shape, whose lower edge defines a surface which coincides with the surface of the working field during the trajectory of the recoater blade over the working field, and at least one roller, comprised of a mechanical element, elongated in shape, placed in parallel to the pusher, downstream of said pusher, and placed in rotation by a drive mechanism around an axis parallel to the pusher, with said recoater blade(s) placed in movement in parallel to the working field through coating phases undertaken by means of control and drive mechanisms.
of means to move the volumes already transformed with respect to the working field.
potentially, a container to hold the material.
of means to control the various elements of the device.
of a raw material feed mechanism.
In order to easily obtain the formation of an extruded log of raw material, rolling on its own axis near the lower edge of the pusher and downstream of said pusher, it is of interest to place the roller, with respect to the pusher, in such a fashion that it ensures the existence of a sufficiently large space between the surface of the previously placed layer and the lower portion of said roller. In fact, such a space allows the material located downstream of the pusher, near the lower edge of the roller, to circulate along the circular path imposed by the movement of the roller, without being xe2x80x9cstoppedxe2x80x9d or deviated from its trajectory, by the fixed material in the lower layers (in particular, the transformed portions) with which it comes in contact. This local looping flow around the axis of the roller, downstream of the pusher, allows for the initiation of the desired formation of the material log, and when its is complete, the maintenance of its rotating movement. It can be confirmed that with a space at least equal in height to the thickness of the layer to be placed, the formation of the log and its rotation are simple to obtain with pastes of limited thickness; for thicker pastes, it is preferable to have a space significantly greater than the thickness of the layer being placed. In order to determine this minimum height of the space (thickness of the layer being placed), it is sufficient to place the axis of rotation of the roller at a height greater than that of the lower edge of the pusher, the difference in height between the roller""s rotational axis and the lower edge of the pusher being at least equal to the value of the largest radius of the circles described by the material points of the roller during its rotating movement.
The device embodying the technique in accordance with the invention may be such that the roller""s rotational axis is determined in relation to the lower edge of the pusher, downstream of said pusher, at a height greater than or equal to the largest radius of the circles described by the material points of the roller during its rotating movement.
As an advantage, the roller""s direction of rotation is in the xe2x80x9crolling direction,xe2x80x9d which allows it to provide the material log being transported a direction of rotation in the xe2x80x9crolling direction.xe2x80x9d
It is of interest to choose a pusher whose side facing the roller presents a protuberance in. parallel to the lower edge of the pusher, with said protuberance presenting a marked angular portion. In fact, this creates a type of cavity in the lower portion of the pusher, which, combined with the action of the roller and the movement of the pusher, contributes to the channeling of the material transported by the recoater blade in the form of the rolling log. It is also of interest to be able to easily adjust the height of said protuberance, for example with the help of a plated part added to the downstream side of the pusher, as illustrated in the attached FIG. 2. In fact, it has been confirmed that this parameter (height of the protuberance) allows for the optimization of the coating quality performed as a function of the Theological characteristics of the treated material. Consequently, with such a xe2x80x9cgeometrically variablexe2x80x9d pusher, optimal treatment can be provided for a wide variety of the pastes, by a simple mechanical adjustment, rather than having to change the pusher with each change of material.
It is advantageous to choose a pusher that presents a tapered lower edge. In fact, it has been confirmed that the very thick pastes, the quality of the layers was significantly improved by the implementation of a pusher with a relatively sharp lower edge. It is of interest for this sort of cutting tool, to experiment with the cutting and clearance angles in order to optimize it for the specific characteristics of each material. With pastes, it is often difficult to ensure a perfect homogeneity of the material, which may restrict the technique. For example, in the case where air bubbles are caught in the paste during mixing, this may lead to the appearance of xe2x80x9cholesxe2x80x9d after the material is spread. In order to resolve this problem, it is of interest to have the recoater blade cover a certain start-up path outside the working field, but near its border. In fact, during this start-up path, prior to entering into continuous operation, the recoater blade in accordance with the invention blends the material located downstream of it and this blending has the effect, on the one hand, of homogenizing the material initially delivered, and on the other, of shaping the materials so it obtains the desired log form.
In the event that large surfaces are to be coated, with a significant thickness of material, a large volume of material must be provided form the start, which makes the initial blending operation more difficult. In addition, the volume of material progressively diminishes during the coating operation, which may, in certain cases, require a progressive adaptation of the operating parameters. Such an adaptation may be easily undertaken with respect to the rotation speed of the roller and the speed of movement of the recoater blade, but providing the means to progressively change the geometric parameters of the recoater blade configuration during the recoating is much more delicate.
The addition of a second dynamic roller with independent rotation allows for the improvement of the operation of the recoater blade in accordance with the invention. In fact, with a recoater blade comprised of a pusher and at least two rollers, a more efficient blending may be obtained than with a single roller, and the effects linked to the evolution of the volume of transported material can also be attenuated. In fact, the volume of the material in circulation near the roller closest to the lower edge of the pusher changes little. This change is transferred to the second roller, which, since it is farther away from the lower edge of the pusher, has little impact on the coating operation.
When the volume of transported material is significant, it is not easy to channel the flow of material with a single roller, especially if a movement of the log approaching a non-sliding rolling movement over the working field is desired. This is why it is advantageous for the embodiment of the recoater blade, to use at least two rollers, which have rotational movement in the xe2x80x9crolling direction.xe2x80x9d
For the embodiment of the recoater blade in accordance with the invention, numerous variations may be implemented: the geometric form of the cross-section of the pusher may or may not be constant along the pusher, the roller may be selected in the family of prismatic bars with circular, square or triangular cross-sections, the number of rollers may also be greater than two, several recoater blades may be associated to perform simultaneous passes (initial shaping or finishing, for example) over the same area, etc.